Kython from the first sentance your army is Illegal, Grey Knights and Requistioned Space Marines can't be used in the same force.
bah.
it's illegal unless the space marines are the base force.
then I am free to field up to 1 HQ, 1 Elite, 2 troops, and 1 Fast Attack from Deamonhunters.
you can even look it up: page 21 of the Daemonhunters codex, there's also a picture of such on page 33.